As a solar cell with high conversion efficiency, a heterojunction solar cell having an amorphous silicon thin-film on a surface of a single-crystalline silicon substrate is known. In a heterojunction solar cell, an intrinsic silicon-based thin-film is inserted between a single-crystalline silicon substrate and a conductive silicon-based thin-film to attain a passivation effect such as termination of defects (mainly dangling bonds of silicon) present on a surface of the crystalline silicon substrate with hydrogen. Thus, carrier recombination on the surface of the crystalline silicon substrate is suppressed, so that photoelectric conversion efficiency can be improved.
A method has been proposed in which a surface of a crystalline silicon substrate or an intrinsic silicon-based thin-film formed thereon is exposed to hydrogen plasma (hydrogen plasma treatment) for the purpose of further improving the conversion efficiency of a heterojunction solar cell. For example, Patent Document 1 suggests that before an amorphous silicon-based thin-film is formed on a crystalline silicon substrate, a surface of the crystalline silicon substrate is subjected to a hydrogen plasma treatment to clean the substrate surface.
Patent Document 2 suggests a method in which an intrinsic amorphous silicon thin-film having a thickness of 1 to 10 nm is formed on a crystalline silicon substrate, and then a hydrogen plasma treatment is followed by formation of a remain thickness portion of the intrinsic amorphous silicon thin-film. When an intrinsic amorphous silicon thin-film is formed in a part of the total thickness, and a hydrogen plasma treatment is then performed as described above, a surface of a crystalline silicon substrate is exposed to hydrogen plasma through the silicon thin-film, and therefore defects on the substrate surface can be cleaned off while plasma damage to the surface of the crystalline silicon substrate is suppressed.
However, when the intrinsic amorphous silicon thin-film is subjected to a hydrogen plasma treatment, the surface of the amorphous silicon film may be etched and damaged depending on the condition of the plasma treatment. Patent Document 3 and Patent Document 4 suggests that a hydrogen plasma treatment is performed not only after formation of an intrinsic amorphous silicon thin-film in a part of the total thickness, but also after the entire thickness portion of the intrinsic amorphous silicon thin-film is formed and before formation of a conductive silicon thin-film.
Patent Document 3 suggests that when a hydrogen plasma treatment is performed after formation of the entire thickness portion of the intrinsic amorphous silicon thin-film, the interface subjected to the hydrogen plasma treatment is restored, so that the film quality of the whole intrinsic amorphous silicon thin-film is improved, and therefore further improvement of conversion efficiency can be expected. Patent Document 4 suggests that not only a passivation effect is attained by a hydrogen plasma treatment, but also the passivation effect is improved by performing chemical vapor deposition (CVD) while introducing hydrogen in an amount of 2 to 6 times the amount of a source gas such as silane during formation of the intrinsic amorphous silicon thin-film.